1. Field of the Invention
The present invention relates to a wavefront aberration measurement apparatus, an exposure apparatus, and a method of manufacturing a device.
2. Description of the Related Art
A projection exposure apparatus which transfers a pattern formed on a mask (e.g., a reticle) onto an exposure target has conventionally been employed to manufacture, by a photolithography process, devices, for example, semiconductor devices such as an integrated circuit and a large-scale integration, image sensing devices such as a charge-coupled device (CCD), display devices such as a liquid crystal panel, and magnetic heads. Since the exposure apparatus must precisely transfer the pattern on a reticle onto an exposure target at a predetermined magnification, it is important to use a projection optical system that has small aberrations and thus is excellent in imaging performance. This is especially important in recent years because the transfer patterns are becoming more sensitive to the aberration of an optical system in response to a demand for further advances in micropatterning of semiconductor devices. Under the circumstance, there has arisen a demand for correctly calculating the optical performance (e.g., the wavefront aberration) of a projection optical system with high accuracy.
One conventional method actually prints a mask pattern on a wafer and observes and inspects the obtained resist image by, for example, a scanning electron microscope (SEM). This method requires a long time for inspection including, for example, exposure and development, and has poor inspection reproducibility due to difficulties in operation of the SEM and errors based on the resist coating and development. In order to solve these problems, a point diffraction interferometer (PDI) having a pinhole to form an ideal spherical wave, and a shearing interferometer which exploits shearing interference have conventionally been known. Nowadays, a measurement apparatus which uses a line diffraction interferometer (LDI) having a slit to form an ideal cylindrical wave or an ideal elliptical wave has been proposed as well. Japanese Patent Laid-Open Nos. 2000-146705, 2000-097666, and 2005-244126, for example, disclose details of a PDI, a shearing interferometer, and an LDI.
When a wavefront aberration measurement apparatus mounted on an exposure apparatus is an LDI, a measurement mark (window/slit member) formed from a slit shape to form an ideal wavefront and a window shape to transmit a wavefront containing the aberration information of a projection optical system is formed on a plate disposed below the projection optical system. Also, a two-dimensional light-receiving element is disposed immediately beneath the above-mentioned plate. The two-dimensional light-receiving element senses interference between two wavefronts generated by the above-mentioned window/slit member. A wavefront aberration generated in the projection optical system is calculated by analyzing the sensed interference fringe image. The position of a lens group which constitutes the projection optical system is corrected based on the calculated wavefront aberration (the aberration values of the respective terms of Zernike polynomials), thereby maintaining the optical performance of the projection optical system with high accuracy.
However, if dirt is adhering on the measurement mark or the measurement position is shifted, no interference fringes are generated in the entire range of the numerical aperture (NA), so an interference fringe image in which fringe information has local defects is obtained. In this case, the wavefront aberration (the aberration values of the respective terms of Zernike polynomials) calculated from the above-mentioned interference fringe image has errors. When the projection optical system is corrected based on the erroneous aberration values, this results in deterioration in optical performance. The above-described discussion is not limited to an LDI and holds true for all optical performance measurement apparatus, which measure the optical performances of optical systems to be measured, such as a PDI and a shearing interferometer.